Electronic clocks commonly use the alternating current (AC) power line mains for an accurate time base as well as for their supply of operating power. Such clocks may typically be incorporated in electronic equipment such as video cassette recorders (VCR's), as timers for controlling timed operation and as timepieces for providing a display of the time of day.
Generally, it is desirable to maintain time-keeping during an AC power failure, particularly in a timer controlling the operation of other apparatus. Standby power typically may be provided by a battery or, preferably, a large capacitor. Although in practical terms, a battery can generally provide a greater energy supply than a capacitor of comparable size and cost, batteries are not desirable, particularly in domestic AC line powered apparatus, since periodic replacement may be needed. This is a nuisance and requires access to internal circuit connections which is not desirable from a user safety aspect.
Many timers and clocks, particularly such as are incorporated in modern electronic equipment such as VCR's, employ a microcomputer. During standby periods, the clock oscillator associated with the microcomputer can be used as a reference source for time-keeping. A disadvantage of such a system is that, in order to keep time when AC power has failed, the microcomputer has to run continuously, thus consuming relatively high operating power. As a result, a relatively large standby energy storage capability is needed, so that time-keeping can only be maintained for a relatively short period of time. Furthermore, accurate frequency control is also required in the absence of AC power, and is typically obtained by using a crystal.